PURPOSE: To minimize the production of defective paper at the stage to vary the production grade of paper to paper of different pulp mixing ratio during the continuous operation of a paper machine.

CONSTITUTION: The amount of the pulp mixture VZ necessary for producing the remaining paper 12 of the same kind by a paper machine 11 is calculated, the amount of the retained pulp mixture VP in a mixer 4 and a buffer tank 9 is detected and the operation of the mixer 4 is stopped when the sum of the pulp mixture (VZ+VO) of the necessary pulp mixture VZ and the marginal pulp VO is decreased below the amount of the retained pulp mixture VP. The mixer 4 is restarted at the new mixing ratio when the amount of the retained pulp mixture VP is decreased below the marginal amount of the retained pulp VO.